Three years ago, Apple unveiled the first iPad, which had a bigger impact than anyone could have imagined. PC and laptop sales have been dropping, and for the first couple of years, sales of iPads regularly eclipsed the most optimistic estimates.

But as Apple prepares to announce new versions of its iPads today, analysts are increasingly pessimistic about the outlook for growth in tablets.

The bottom is certainly not falling out. But it appears analysts believe the era of furious growth for tablets, and the iPad in particular, are over.

The drama coming out of Apple's event on Tuesday is whether the new version of the big iPad and the iPad Mini can supercharge growth again.

"It sounds jaded, but tablets are a maturing market: We expect few surprises in this category that Apple kick-started only three years ago," said Forrester Research analyst Sarah Rotman Epps. "It will be difficult for Apple to move the needle on new tablet sales, as the strongest growth is coming from emerging markets where customers are more price-sensitive."

This is a big shift for the iPad. When Steve Jobs first displayed the device on stage in early 2010, there was a bit of head scratching. It's nice, but who is going to buy the darn thing?

Turns out, plenty of people. In 2012, Gartner says about 120 million tablets were shipped. The growth had been tremendous. And as a result, other tech companies like Microsoft and Intel and Dell and Hewlett-Packard, which built their businesses on Windows machines, found themselves blindsided and scrambling to respond.

But this summer, iPad sales showed some cooling. Sales in Apple's third quarter fell to 14.6 million units, down from 19.4 million units the previous quarter and 17 million for the same quarter a year earlier.

Apple will release sales of iPads for its fourth quarter, which ended Sept. 30, Monday when it reports earnings.

Apple wasn't alone. Worldwide sales of tablets fell overall. And many analysts chalked this up to Apple's decision to wait several months longer than usual before introducing new iPad versions.

Perhaps. But more recently, as analysts have published iPad sales estimates for the coming years, there seems to be a noticeable lack of optimism.

Katy Huberty, a Morgan Stanley research analyst, projects that sales of iPads for fiscal 2013, which ended in September, will generate $31.4 billion in revenue. By 2016, she projects iPad revenue to grow to only $33.9 billion.

That's not bad for a business that didn't exist four years ago. But after growing 83% in 2011 and 71% in 2012, it's still a big comedown to see growth fall into single digits, even if it is from a much larger base.

Part of the issue at the moment is that with introduction of the iPad Mini, Apple's probably cannibalizing a bit of its own revenue growth.

Ben Reitzes, an analyst at Barclay's Capital, projects that the number of overall iPads sold annually will increase by just 2% in the fiscal year ending in September 2014.

In a note to clients, Reitzes worried that Apple is becoming increasingly dependent on iPhone revenue. Even with the iPad's success, the iPhone has grown from 42% of Apple's revenue in 2011 to a projected 53% of revenue in fiscal 2013. If iPad sales growth continues to slow, that concentration will become even higher.

"We believe one of Apple's big issues right now is that it is considered a 'one product' story again,” Reitzes wrote in August.

Perhaps because of this growing skepticism about the iPad's future prospects, Cantor Fitzgerald analyst Brian White says there is a lot riding on the iPad announcements Tuesday to show that consumers are still eager to snap up new versions of the devices.

"We believe this will be the most important refresh of the iPad franchise for Apple since the first iPad went on sale on April 4, 2010," White wrote in a note to clients. "We believe this iPad launch will help put a bottom in the iPad sales cycle that reached negative territory in 3Q:FY13 for the first time in the history of the iPad."

But Epps of Forrester said she thinks it's going to be tough for Apple to get consumers excited based on improved features or designs.

She said adding a Retina screen to the iPad Mini will help. But she's also projecting that worldwide sales of new tablets will only grow from 143 million units in calendar year 2013 to 166 million units.

Making this more complicated is that Apple has been steadily losing market share in tablets, as other vendors selling Android-based tablets have finally started offering viable alternatives. Apple had a 60.3% market share at the end of the June 2012 quarter. By June 2013, that had fallen to 32.5%.

It's worth noting that Apple has been emphatic about the fact that it doesn't worry a bit about market share.

But Epps said that as with smartphones, Apple is turning its back on some of the fastest-growing areas for tablets, such as emerging economies.

"As we’ve seen with recent iPhone sales, Apple has an easier time selling high-end products like the 5s to existing customers than selling less-expensive-but-not-cheap products like the 5c to new customers," Epps said. "The low end of the market doesn’t belong to Apple, and they haven’t aggressively tried to change that."

Of course, Apple is used to proving the skeptics wrong.

And now Apple will have new iPads to tout just in time for the holiday shopping season. Its best bet for silencing the doubters this time is to post a strong couple of quarters of iPad sales.